1. Field of the Invention
The present invention relates to a sander and, in particular, to a manually operated sander, including a rectangular babe plate, a sanding pad attachable to a front side of the base plate, and a locking mechanism for releasably attaching the sanding pad to the base plate and including at least one pawl provided on each of the front side of the base plate and the sanding pad and having a contact surface, with attachment and detachment of the sanding pad to and from the base plate being effected by respective relative movements of the contact surfaces of the respective pawls in opposite directions.
2. Description of the Prior Art
With the sanders of the type described above, for treating workpieces, an abrasive or emory paper is used. The abrasive paper can be releasably secured to the sanding pad, e.g., with a mechanical locking device or with VELCRO(copyright) fastening means. When the VELCRO(copyright) fastening means is used, the sanding pad should he provided with a surface specifically adapted for attaching a VELCRO(copyright) element thereto. When the VELCRO(copyright) fastener is not used for attaching the abrasive paper to the sanding pad, no specific treatment of the respective surface of the sanding pad is necessary. In addition, the side of the sanding pad adjacent to a workpiece may be formed by an elastic material, e.g., a foam material. Such a sanding pad is subjected to wear and with time needs to be replaced.
Conventionally, the sanding pad is secured to the front side of the base plate with screws, which permits to release the sanding pad when necessary. To prevent loosening of the screws, as a result of vibrations, during sanding of a workpiece, the screws must be tightened very strongly.
German Utility Model DE-U-200 03 064 disclosed a sander in which the sanding pad is secured to the base plate by a locking mechanism, The locking mechanism including at least one pawl provided on each of the front side of the base plate and the sanding pad and having a contact surface. The attachment and detachment of the sanding pad to and from the base plate are effected by respective movements of the contact surfaces of the respective pawls in opposite directions against a biasing force,
The advantage of the connection of the sanding pad with the base plate according to DE-U-200 03 064 consists in that it can be effected relatively simply with a locking mechanism released by a user of the sander without using any tool. Such a connection prevents the user from attaching to the base plate of an unsuitable or mismatched sanding pad, without a suitable tool being available.
However, a drawback of the known connection consists in that occurring high centrifugal forces cannot be adequately absorbed by the connection or attachment means. This results in a high load acting on the locking mechanism, which significantly reduces its service life.
Further, the locking mechanism with spring biasing means complicates its handling by a user. Specifically, the force, which the user has to apply to overcome the biasing force of the spring means which has to be matched to the possible high centrifugal forces, is rather high. This makes the pad replacement more difficult.
Accordingly, an object of the present invention is to provide a sander with a locking mechanism for releasably attaching a sanding pad, which can be easily handled.
Another object of the present invention is to provide a sander with a locking mechanism capable to withstand high centrifugal forces.
A further object of the present invention is to provide a sander with a locking mechanism that would provide for a cost-effective manufacturing of the entire sander.
These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a sander with a locking mechanism of the above-describe type in which the contact surfaces of the pawls have a longitudinal extent a length of which corresponds to from 0.2 to 1.3 times of a width of the front side of the base plate.
A big length and, thereby, a large contact surface of the pawls insure that a large centrifugal force, which is produced between the sanding pad and the base plate, is absorbed without any problem, as the greater part of the centrifugal force is distributed between the surfaces of both the base plate and the sanding pad. The distribution of the centrifugal force between the base plate and the sanding pad permits to form lighter base plate and sanding pad, as a point application of forces does not exist. Large surface area of the contact surfaces facilitate their adjustment and significantly facilitates handling of the locking mechanism. The contact surfaces of the pawls can be formed as circular or elongate surfaces.
According to a preferred embodiment of the present invention, to insure a most even distribution of the centrifugal forces between the sanding pad and the base plate, the contact surfaces extend longitudinally from the periphery of the front side to its center. This ray-liked arrangement of the pawls or their contact surfaces can be easily produced and, therefore, this arrangement of the contact surfaces is very cost-effective. In particular, with orbital sanders, the discussed above arrangement of the contact surfaces insures an optimal releasably attachment of the sanding pad to the base plate.
Advantageously, there are provided four contact surfaces on each of the base plate and the sanding pad, with adjacent contact surfaces forming an angle of about 90xc2x0 with each other. This provides for absorbing centrifugal forces in all directions. This is particular important with orbital sanders because the load in these sanders is not applied from a single direction. Further, with four contest surfaces arranged in the above-discussed manner, an automatic centering takes place during the attachment of the sanding pad to the base plate. The automatic centering significantly facilitates handling of the inventive sander in comparison with conventional sanders. Moreover, the centrifugal forces acting on the sanding pad produce a wedge effect with respect to the correspondingly pawls as the adjacent pawls and the contact surfaces form a V-shaped arrangement. At the same time, the about 90xc2x0 -arrangement of the pawls permits to release the sanding pad from the base plate without application of substantial forces.
To provide for a simple and rapid replacement of the sanding pad, the locking mechanism includes an adjusting ever for displacing at least one pawl and its contact surface between engaging and release positions of the locking mechanism. The adjusting lever can be actuated with one hand. It can be arbitrary formed. E.g., the adjusting lever can be formed as a knob or as a wing-shaped engagement element. Advantageously, the adjusting lever is supported on the base plate. However, the adjusting lever can be supported at another location, e.g., on a housing to which the base plate is secured. The respective pawl advantageously can be provided with an elongate opening in which a bolt provided on the adjusting lever, engages. This insures a cost-effective manufacturing of the locking mechanism according to the present invention. Further, die inventive locking mechanism provides for an increased service life in a very dusty environment.
Advantageously, there are provided two adjustable contact surfaces on the same pawl in order to insure a reliable attachment of the sanding pad to the base plate. With four contact surfaces, simple replacement of the sanding pad is insured because in the release position, only two of the four contact surfaces are in an engagement position.
The adjustable contact surfaces are advantageously provided on a pawl formed of metal which insures, on one hand, a sufficiently high stability and, on the other hand, a high elasticity. Moreover, a small friction resistance of metal facilitates displacement of the contact surfaces into the release position, which further facilitates handling of the sander.